As a method to obtain a three dimensional image from two stereoscopically photographed films have been known the lenticular method, polarization method, complementary color method, etc by which stereoscopic observation of the three dimensional image is permitted.
Conventionally, the distances in the directions of three dimensional coordinates (axes x, y and z) has been calculated or measured with respect to the three dimensional image which was prepared by the method described above. This procedure was carried out by the following methods, that is, a method of measuring and magnifying the distance on a film or printing paper, a method for measurement by plotting the coordinates of axes X and Y of a scale reduced image on a certain appropriate section paper and diagramming the image in reference to the known and pre-calculated points or a method of measuring the distance based on an image on an optical film. In all these conventional methods, it was necessary to set the known points of the subject on a photographic image to be used and to measure in advance the distances of the above points in the directions of axes x, y and z on the subject. Accordingly, in case of measuring the internal positions of the subjects such as a brain and metallic cast product, that is, the position or distances among several points in the three dimensional space, it was impossible to measure an accurate distance since previous setting of the points which were the reference points for measurement and previous measurement of the distances of the above points in the directions of axes x, y and z were impossible.
On the other hand, it is possible to prepare a three dimensional image and to practise these dimensional measurement which permits accurate measurement of the distance and position on an actual subject by measuring the distance and position of the points on the above three dimensional image; therefore a clear and precise three dimensional image has been required. However, the conventional projector to obtain the three dimensional image is constructed as shown in FIG. 1 and has been unable to provide a clear and precise three dimensional image. In other words, as shown in FIG. 1, the left side film stand 3 and the right side film stand 4 on which the left side film 1 and the right side film 2 stereoscopically photographed are set are required, the left side condenser lens 5 and the right side condenser lens 6 such as Fresnel lenses are arranged below the left and right side film stands 3 and 4 and furthermore the left side light source 7 and the right side light source 8 are provided below the condenser lenses 5 and 6. The left side lens 9 and the right side lens 10 are arranged at the focal positions of the condenser lenses 5 and 6 above the film stands 3 and 4, and the left side first mirror 11 and the right side first mirror 12 which are inclined to 45.degree. to change the optical axis to the horizontal direction are arranged to oppose each other above lenses 9 and 10. The left side second mirror 13 and the right side second mirror 14 are slidably provided at the intermediate positions between the left side first mirror 11 and the right side first mirror 12 to change the direction of the optical axis to the above and the third mirror 16 is arranged to lead the light from the second mirrors 13 and 14 to the screen 15. The left side polarizing filter 17 and the right side polarizing filter 18 which polarize the light passing through the films 1 and 2 are vertically arranged between the first mirrors 11 and 12 and the second mirrors 13 and 14. In the conventional apparatus constructed as described above, when the images of the left side film 1 and the right side film 2 are projected onto the screen 15, the images deviate from each other as shown in FIG. 1. If the second mirrors 13 and 14 are slid to adjust the angle to completely overlap the images and to eliminate such deviation, the images are distorted toward the right and left side peripheries. When such image is viewed through the polarizing glasses, only a three dimensional image with distortion and out-of-focus part can be observed and it cannot be used for accurate three dimensional measurement. In other words, this is because the measured points which are photographed on the right and left side films do not agree on the three dimensional image or are projected deviated from the original positions as the magnification and contraction ratios differ at each part of the image.